More than enough clean energy to supply all of humanity's needs falls on the Earth every day in the form of sunlight. The problem is how to capture this energy. Much of the solar energy is translated into evaporation of seawater to produce clouds, rain, and wind. Some of this can be captured by hydroelectric dams and windmills. Some is captured by plants through photosynthesis. Most is simply reflected back into space.
One problem with solar energy is that it is spread thinly over the surface of the earth. The energy density of sunlight is about Capturing a lot of energy requires a large area. Thus, solar cells are an expensive because it takes so many of them. Solar cells cost about $1000 per square meter. Solar power generators using mirrors are complex and require a large field of reflectors. Perhaps the most practical way to capture solar energy is through photosynthesis.
Photosynthesis is a process by which carbon atoms are absorbed from CO2 molecules in the atmosphere, and oxygen molecules are released. When plants (or their fossilized byproducts such as oil or gas) are burned, oxygen molecules are absorbed and CO2 molecules are released back into the atmosphere. Burning also releases the energy that was stored in the plant by photosynthesis. Burning fossil fuel releases CO2 that was absorbed millions of years ago, and produces a net increase in atmospheric CO2. However, burning recently grown biomass releases recently absorbed CO2. This is a carbon neutral cycle. There is no net increase in greenhouse gases. Thus, fuel from biomass is a carbon neutral energy source.
Biofuels are safe to store, easy to transport, and clean to burn. Bidies nearly as energy dense as gasoline, and much less toxic when spilled. And there are no toxic materials or heavy metals involved such as are present in the batteries used in hybrid and electric vehicles. Estimates are that conversion of cellulose to biofuels will become commercially viable within two to five years. It could become carbon neutral within a decade or two.
In the long term however, biomass production cannot be a long-term replacement for oil, coal, and natural gas until several more fundamental problems are solved.
The biggest problem lies in the vast amount of biomass required and the limited amount of unused real estate that is appropriate for biomass production. Most of the world's best farmland is already under cultivation for food crops, and using the Earth's remaining forests and wetlands for biomass production is not an environmentally sound solution. Although the amount of land under cultivation can be increased, and increases in agricultural production can be expected from genetic engineering and improved fertilizers, increased acreage will grow ever more costly as population growth transforms farmland into cities and suburbs. Doubling the amount of land under cultivation is probably not possible.
A second and related problem is that demand for fuel will drive up the price of food in the marketplace so long as food and fuel compete for the same cropland. Ethanol production from corn has already had a significant impact on price of corn and products made from corn. The effect of rising prices is spilling over into other cereal crops such as wheat, soybeans, and even sorghum.
A third problem is that there is a limited supply of water in most regions of the world not already under cultivation. Major irrigation projects are enormously expensive, and often are destructive to the environment. Water for irrigation is subject to seasonal variations and drought. Water is already a limiting resource in many parts of the world.
In the near term, current methods of farming for biomass require fossil fuel for plowing, planting, and harvesting. Until the fuel for biomass production is derived from the biomass produced, this process is not carbon neutral.
Finally, there is a limited growing season in regions more than 50 degrees from the equator.
Until these problems are overcome, fuel from biomass cannot become a long-term alternative to fossil fuel.